You may not be able to.
A rather extreme statement, particularly from an organization that does program evaluation. Nonetheless, we stand by it. It’s hard to get too far into a discussion of the notion of proof without talking at least a bit about Karl Popper’s philosophy of science and his key idea of falsifiability. For Popper it is always easy to find evidence (legitimate evidence, I will add) to support a general idea (in this case that your program works). If a single instance can be found where the idea doesn’t hold up however, the idea will be disproven. The problem is that in principle, it is impossible to test every possible instance. There is an infinity of them. We can sometimes try to get around this through the use of inferential statistics, all those t-tests and ANOVAs you remember from your stat course that tell you that 95% of the time, the results you found would not have been due to chance. But what about that remaining five percent? You’re out of luck. For Popper, science, knowledge, only advances as ideas are repeatedly tested by scientists (or evaluators, though the field was in its infancy when Popper wrote The Logic of Scientific Discovery) and remains, in any event, always provisional. A recent article in the New Yorker in fact shows that a few scientists are beginning to question even this. And then there’s this one from the New York Times.